Abstract
Contrary to the thus far accepted knowledge we have found that light enhances hook curvature in many dicots including tomato through low fluence and very low fluence responses of phytochrome. Since hook curvature is a growth response at the apical part, the role of phytohormones was examined. Evolution of endogenous ethylene was not altered by R or FR, which enhances curvature. Application of ACC, an ethylene biosynthesis precursor, or of ethylene to dark-grown tomato seedlings caused hook to open. Treatment with PAC, gibberellin biosynthesis inhibitor, enhanced hook curvature as markedly as R or FR. When auxin transport was blocked by NPA, oppositely no hook was formed in not only non-irradiated seedlings but also R- or FR-irradiated ones. These results suggest that gibberellin and auxin, rather than ethylene, may negatively or positively be involved in hook curvature.
